1. Field of the Invention
The present invention relates to a method of and an apparatus for inspecting the minimum annular width of a land formed on a printed circuit board, and more particularly to an improvement to reconcile accuracy and speed in the inspection.
2. Description of the Prior Art
A printed circuit board is formed with wiring patterns having lands and through holes. Automatic inspection apparatus for the printed circuit board have been developed to automatically inspect the relative formation positions of the wiring patterns and through holes by reading the image of the printed circuit board and have been put to practical use. One of the significant inspection items in the printed circuit board inspection is the minimum annular width of a land which remains after forming a through hole therein.
FIG. 28 illustrates an example of the conventional inspection of an annular width of a land. There is formed a through hole 2 in a land 1. The through hole 2 is deviated from the center of the land 1 resulting from a drilling error and the like. During the inspection of the annular width, the binary images of a wiring pattern including the land 1 and the through hole 2 are obtained. A radial operator 3 is positioned such that its center coincides with the center of the through hole 2. A logical operation is performed between the radial operator 3 and the image of the wiring pattern. The radial operator 3 has a plurality of arms. The annular widths of the wiring pattern in the elongated directions of the respective arms are determined from the logical operation results on the arms. Then, a minimum annular width W.sub.L which is the minimum value among the respective annular widths is obtained. When the minimum annular width W.sub.L is smaller than a threshold value, it is determined that the printed circuit board is defective. Such a radial operator is disclosed in Japanese Patent Application Laid-Open No. 62-263404, for example.
The direction in which the through hole 2 is deviated from the land 1 is not constant and varies in respective printed boards to be inspected. In order to accurately determine the is minimum annular width, it is necessary to increase the number of arms of the radial operator 3. The increase in the number of arms causes the time period required for the logical operation to grow increase so that the inspection is prevented from being performed at a high speed. Another problem is that the costs for manufacturing the inspection apparatus rise.